The invention is related to providing a method of measuring a flow velocity using a phase difference of ultrasonic waves for calculating a flow rate of fluid in a large river or open sluice way channel and a flow rate of liquid or gas in a pipe having a large inner diameter.
The core portion of a recent well-known ultrasonic flow rate measuring system for a large open sluice way channel or a pipe having a large inner diameter is designed to measure a flow velocity of liquid or gas. The system is normally called xe2x80x9ca flowmeter.xe2x80x9d
Most of the flow rates measuring systems are supposed to measure the flow velocity based on a flow velocity measuring method utilizing an ultrasonic transit time difference.
As shown in FIG. 1, a flow velocity measuring system using an ultrasonic transit time difference operates as follows: ultrasonic transducers 1 and 2 for transmitting/receiving an ultrasonic wave are mounted at an angle xcex1 to face each other. A switch circuit 3 functions to switch the ultrasonic transducers 1 and 2 in turns to the inputs of transmitting and receiving circuits. An example of a transmitting and receiving circuit is an ultrasonic pulse oscillator 4 and an ultrasonic receiving signal amplifier 5. Next, a pulse shaping circuit 6 receives an amplified signal and shapes it into a pulse signal of a shorter period. A time interval measuring apparatus 7 measures transit times t1 and t2 at an interval distance L from the transmitting time till the receiving time. An arithmetic logic unit 8 computes a flow velocity based on expression (1).
That is to say, the transit time t1, which the ultrasonic pulse is transmitted from transducer 1 to transducer 2 (as shown in FIG. 1), is measured. On the contrary, the transit time t2, which the ultrasonic pulse is transmitted from the transducer 2 to the transducer 1, is measured. These times measured are made as follows:             t      1        =          L              C        +                  V          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢          α                      ;      xe2x80x83    ⁢            t      2        =          L              C        -                  V          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢          α                    
Therefore, the transit time difference xcex94t that t2xe2x88x92t1 can be presented as follows:                               Δ          ⁢                      xe2x80x83                    ⁢          t                =                              2            ⁢            L            ⁢                          xe2x80x83                        ⁢            cos            ⁢                          xe2x80x83                        ⁢            α            ⁢                          xe2x80x83                        ⁢            V                                C            2                                              (        1        )            
Wherein, C is a sound velocity of liquid or gas, L is an interval between transducers 1 and 2 and V is an average flow velocity in the interval L.
The flow velocity V from the expression (1) is deduced as follows:                     V        =                              Δ            ⁢                          xe2x80x83                        ⁢                          tC              2                                            2            ⁢                          xe2x80x83                        ⁢            L            ⁢                          xe2x80x83                        ⁢            cos            ⁢                          xe2x80x83                        ⁢            α                                              (        2        )            
It may be called xe2x80x9cA Transit Time Difference Flow Velocity Measuring Methodxe2x80x9d, because the flow velocity V is proportional to the transit time difference xcex94t. It seems that the transit time difference flow velocity measuring method is related to the sound velocity, because there is an item C2, which is the square of the sound velocity, in the expression (2). The item C2 of the sound velocity must be simultaneously measured at the time of the flow velocity measurement. The square of the sound velocity is represented as follows:       C    2    =            L      2                      t        1            ·              t        2            
The sound velocity item C2 is substituted into the expression (2) to make the final flow velocity measuring expression as follows:                     V        =                                                            L                2                                            2                ⁢                L                ⁢                                  xe2x80x83                                ⁢                cos                ⁢                                  xe2x80x83                                ⁢                α                                      ·                                                            t                  2                                -                                  t                  1                                                                              t                  1                                ·                                  t                  2                                                              =                                                    L                2                                            2                ⁢                d                                      ⁢                                                            t                  2                                -                                  t                  1                                                                              t                  1                                ·                                  t                  2                                                                                        (        3        )            
Then, the flow velocity is obtainable by measuring only the ultrasonic transit times t2 and t1 and computing the expression (3), because L2/2d=const.
Typical prior arts are disclosed in U.S. Pat. No. 5,531,124 granted on Jul. 2, 1996, Japanese Patent No. 2,676,321 granted on Jul. 25, 1998, Manual of Ultrasonic flow Measuring and Apparatus thereof and Ultrasonic Flowmeter related to Model UF-2000C manufactured by the Ultra flux Co.
The transit time difference flow velocity measuring method has a great advantage in that the flow velocity measuring is simply performed as illustrated in the expression (3), even though the sound velocity is seriously changed in fluid. That is, although the expression (3) seems like being related to the square of the sound velocity according to a deliberative method of the flow velocity measuring expression, it is not principally related to the flow velocity.
For example, the difference between the reciprocal numbers with respect to the transit times t1 and t2 is obtained as follows:                     1                  t          1                    -              1                  t          2                      =                  2        ⁢        V        ⁢                  xe2x80x83                ⁢        cos        ⁢                  xe2x80x83                ⁢        α            L        ,
The items of the sound velocity C are offset to each other. Therefore, the flow velocity V is as follows:   V  =                    L                  cos          ⁢                      xe2x80x83                    ⁢          α                    ⁢              (                              1                          t              1                                -                      1                          t              2                                      )              =                            L          2                          2          ⁢                      xe2x80x83                    ⁢          d                    ⁢              (                                            t              2                        -                          t              1                                                          t              1                        ·                          t              2                                      )            
Wherein, d=L cos xcex1.
As a result, the expression obtained is the same as the one (3).
It has a great advantage in that the transit time difference flow velocity measuring method has no relation with the change in the great range of the sound velocity C in fluid. But, the transit time difference flow velocity measuring method is limited to its use. For example, when the transit distance L is very small and/or the flow velocity V is very low, it is very difficult to measure the flow velocity, precisely. If L=0.05 m, V=0.1 m/s, xcex1=45xc2x0 and C≈1500 m/s, xcex94t≈3.14xc3x9710xe2x88x929 s.
If it is intended to measure a very little time difference within the error range of 1%, the time difference absolute measuring error should not exceed the range of 3xc3x9710xe2x88x9211 s. Measuring the time difference based on such a method needs a relative complex time interval measuring apparatus. Also, an apparatus for catching moments of transmitting/receiving the ultrasonic pulses must be very stable and precise. As mentioned below, the transit time difference flow velocity measuring method causes many problems, when the gas flow velocity is measured in a pipe, or the horizontal flow velocity is measured in a channel or river.
In addition to the transit time difference flow velocity measuring method, an ultrasonic phase difference flow velocity measuring method is also well known. For example, there are Dutch Patent Laid-Open Publication No. DE19722140 on Nov. 12, 1997 and Japanese Patent Laid-Open Publication No. Hei 10-104039 published on Apr. 24, 1998, both of which are entitled as xe2x80x9cA multi-channel flow rate measuring system.xe2x80x9d
FIGS. 2A and 2B show a typical configuration of a phase difference flow velocity measuring system. Ultrasonic transducers 1, 1xe2x80x2 and 2, 2xe2x80x2 are positioned to face each other. A sine wave oscillator 9 generates a sine wave having a frequency ƒ. A phase shifter 10 adjusts the phase of received ultrasonic signals. An amplifier 11 amplifies the received signals from the phase shifter 10 and the transducer 1xe2x80x2. A phase difference discriminator 12 measures the phase difference between the received phase signals. When the sine wave oscillator 9 is operated, the transducers 2 and 2xe2x80x2 transmit ultrasonic waves at the same phase. At that time, the phase signals, which the receiving transducers 1 and 1xe2x80x2 receive are as follows:
xcfx861=2xcfx80ƒxc2x7t1+xcfx860; xcfx862=2xcfx80ƒt2+xcfx860
Wherein,       t    1    =                    L                  C          -                      V            ⁢                          xe2x80x83                        ⁢            cos            ⁢                          xe2x80x83                        ⁢            α                              :              xe2x80x83            ⁢              t        2              =          L              C        +                  V          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢          α                    
xcfx860 is an initial phase that the ultrasonic wave is firstly transmitted. Therefore, the phase difference xcex94xcfx86 between the received signals is as follows:                     Δϕ        =                                            ϕ              1                        -                          ϕ              2                                =                                    2              ⁢              π              ⁢                              xe2x80x83                            ⁢              f              ⁢                              xe2x80x83                            ⁢              Δ              ⁢                              xe2x80x83                            ⁢              t                        =                          2              ⁢              π              ⁢                              xe2x80x83                            ⁢              f              ⁢                                                2                  ⁢                  LV                  ⁢                                      xe2x80x83                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                  α                                                  C                  2                                                                                        (        4        )            
Herein, the flow velocity is as follows:                     V        =                              Δϕ            ⁢                          xe2x80x83                        ⁢                          C              2                                            4            ⁢            π            ⁢                          xe2x80x83                        ⁢            fL            ⁢                          xe2x80x83                        ⁢            cos            ⁢                          xe2x80x83                        ⁢            α                                              (        5        )            
The phase difference method has features in that the ultrasonic waves can be continuously transmitted and the phase difference xcex94xcfx86 is proportional to the frequency ƒ unlike the transit time difference method. Therefore, even if L and V are very small, when the ultrasonic frequency ƒ is selected at a higher one, the phase difference becomes larger, so that the phase difference measuring is conveniently and precisely done.
Also, if L is relatively larger, the damping factor is very small over the ultrasonic pulse, because the ultrasonic continuous waves are transmitted/received. Further, even though the amplitude of the received signal significantly pulsates, the received signal can be sufficiently amplified, because the receiving moment is not measured. And, an automatic gain control circuit can be used in the method. It means that there is not any problem in measuring the phase difference at all. Only, the phase difference method is preferably used under the condition that the sound velocity C is almost not changed or in the case that any other means measures the sound velocity C. For example, in order to measure the gas flow rate, the sound velocity of gas can be easily calculated under the condition that a pressure gauge and a thermometer are mounted in the pipe.
As mentioned above, the great advantage of the ultrasonic transit time difference method can be utilized even under the situation that the sound velocity in fluid is significantly changed. But, if the interval L between the transducers becomes larger, the following problems occur due to the transmitting/receiving of the ultrasonic pulse.
First, the ultrasonic pulse has a larger damping factor over the sine wave because of its sufficient harmonic wave components or overtones. If the ultrasonic transit distance L becomes larger, it is difficult to receive the transmitted ultrasonic wave and the received pulse becomes a bell form due to the serious damping problem.
For all that, it cannot help increasing the ultrasonic wave intensity that can be auxiliarily adjusted. If the intensity becomes higher, the cavity phenomenon occurs in a river, so that the ultrasonic wave is not transmitted. Especially, as the pulse frequency becomes lower in order to reduce the damping factor, the ultrasonic intensity also becomes lower, which causes the cavity phenomenon.
Second, the ultrasonic pulse is not damped only by the distance L in the procedure of being transmitted, but the amplitude of the ultrasonic wave seriously pulsates, by which the ultrasonic wave is diffused and reflected because of various sizes of eddy currents, the concentration change of floating particles, the temperature change of water, etc. in the open sluice way channel. It sometime happens that the ultrasonic wave is not received.
When the flow velocity of gas is measured, the damping factor of the ultrasonic pulse is larger than that in liquid. The serious damping and pulsation of the ultrasonic pulse cause many errors, when it is subjected to catch the moment that the ultrasonic pulse reaches. Thus, the flow velocity measuring error is increased.
Due to these reasons, the ultrasonic transit distance L is limited in that the ultrasonic pulse is transmitted/received and the flow velocity is measured based on a time difference method. Thus, it has great difficulty in measuring the flow velocity in the larger open sluiceway channel or river and a larger pipe.
If the phase difference method is used for measuring the flow velocity, its damping factor is decreased two or three times over that of the ultrasonic pulse, because the ultrasonic continuous waves (sine waves) are transmitted/received. Also, the phase difference method is not relevant to the amplitude pulsation of the received signals, because it is not related to catching the moment that the ultrasonic pulse reaches, but the phase difference between two sine waves is measured. Nevertheless, the phase difference method is limited to its use. If the phase difference xcex94xcfx86 between two sine waves is equal to mxcfx80+xcex2, a general phase difference measuring apparatus cannot detect m (1, 2, 3, . . . ). If the ultrasonic transit distance L or the flow velocity V is larger, xcex94xcfx86 becomes greater than xcfx80. For example, if it is intended to measure the flow rate of gas in the pipe having an inner diameter "PHgr" of 300 mm, the cross-sectional average flow velocity V of gas is generally 10-30 m/s. Then, assuming that the sound velocity C is 400 m/s, the ultrasonic frequency ƒ is selected at 400 KHz in order to be beyond the frequency band of noises and an angle xcex1 is 45xc2x0, the changing width of the phase difference xcex94xcfx86 is as follows:
xcex94xcfx86=9.42xcx9c28.26rad≈(2xcfx80+0.998xcfx80)xcx9c(8xcfx80+0.995xcfx80)
That is, xcex94xcfx86 greater than xcfx80.
If L=10 m, V=3 m/s, ƒ=200 KHz and C=1500 m/s in a relatively smaller open channel, the phase difference xcex94xcfx86 is as follows:
xcex94xcfx86≈16.746rad=5xcfx80+0.33xcfx80 greater than xcfx80.
Thus, the phase difference method cannot be used in measuring the flow velocity in the relatively smaller open channel. In other words, the transit time difference method has an advantage in being used under the situation that the sound velocity is changed to a larger range. But, it has disadvantages in that if the flow velocity-measuring interval L is larger, the ultrasonic pulse becomes unstable, because the ultrasonic pulse is greatly damped due to its own property during the transmitting/receiving.
The phase difference method has advantages in that the damping factor is relatively smaller and the received signal is easily processed, because the ultrasonic sine wave is transmitted/received. But, if the phase difference exceeds xcfx80 radians by which the interval L and the flow velocity V is larger or the sound velocity is lower, it is not possible to measure the flow velocity based on the phase difference method. Also, the phase difference method has a disadvantage in that the sound velocity should be separately measured.
An object of the invention is to provide an ultrasonic flow velocity measuring method for measuring a flow velocity based on a phase difference method, smoothly, if a flow velocity measuring interval L is relatively larger, for example if a horizontal average flow velocity is measured in an open sluice way channel or river.
The other object of the invention is to provide an ultrasonic flow velocity measuring method for measuring the flow velocity based on a phase difference method, smoothly, if a flow velocity measuring interval L is relatively larger, for example if a gas flow velocity is measured in a pipe of a relatively larger inner diameter.
Another object of the invention is to provide an ultrasonic flow velocity measuring method for measuring the flow velocity based on a phase difference method, smoothly, if a gas or liquid flow velocity is measured in a pipe of a relatively larger inner diameter.
Still another object of the invention is to provide an ultrasonic flow velocity measuring method for measuring the flow velocity based on a phase difference method, smoothly, if a flow velocity is relatively larger and the sound velocity is relatively lower.
According to the invention, an ultrasonic flow velocity measuring method based on a phase difference method not depending upon a sound velocity, comprises: amplitude-modulating a continuous ultrasonic wave of a carrier frequench ƒC into at least one signal of an amplitude-modulation frequench ƒM, if a phase difference between the ultrasonic waves transited in directions similar and contrary to the flow velocity exceeds xcfx80 radians beyond the measuring range of a general phase difference discriminator and becomes mxcfx80+xcex2, and transiting/receiving the amplitude-modulated signals in directions similar and contrary to the flow velocity; obtaining m using the signal of amplitude-modulation frequench ƒM, measuring a part of axcfx80 less than xcfx80, obtaining the phase difference between the signals of the carrier frequench ƒC and obtaining the total phase differences xcex94xcfx86C, precisely, thereby enabling the very accurate measurement of the phase difference between the ultrasonic waves.